1. Field of the Invention
The present invention relates to a method of manufacturing a silver halide photographic emulsion for providing a light-sensitive material with high sensitivity and low fogging density. The present invention also relates to a method of manufacturing a silver halide photographic emulsion for providing a light-sensitive material whose sensitivity and fogging density do not vary much upon storage.
2. Description of the Related Art
Basic properties required for a photographic silver halide emulsion are high sensitivity, low fogging density, and fine graininess.
In order to increase the sensitivity of an emulsion, (1) to increase the number of photons absorbed by a single grain, (2) to increase the efficiency of converting photoelectrons generated by light absorption into a silver cluster (latent image), and (3) to increase development activity for effectively utilizing the obtained latent image, are required. Increasing the size increases the number of photons absorbed by a single grain but degrades image quality. Increasing the development activity is an effective means of increasing sensitivity. In the case of parallel development such as color development, however, the graininess is generally degraded. In order to increase the sensitivity without degrading graininess, it is most preferable to increase the efficiency of converting photoelectrons into a latent image, i.e., increase a quantum efficiency. In order to increase the quantum efficiency, a low-efficiency process such as recombination and latent image dispersion must be minimized. It is known that a reduction sensitization method of forming a small silver nucleus without development activity inside or on the surface of a silver halide is effective to prevent recombination.
The method of reduction sensitization has been studied for a long time. Carroll, Lowe et al., and Fallens et al. disclose that a tin compound, a polyamine compound, and a thiourea dioxide-based compound are effective as a reduction sensitizer in U.S. Pat. Nos. 2,487,850 and 2,512,925 and British Patent 789,823, respectively. Collier compares properties of silver nuclei formed by various reduction sensitization methods in "Photographic Science and Engineering", Vol. 23, P. 113 (1979). Collier adopted methods of dimethylamineborane, stannous chloride, hydrazine, high-pH ripening, and low-pAg ripening. Reduction sensitization methods are also disclosed in U.S. Pat. Nos. 2,518,698, 3,201,254, 3,411,917, 3,779,777, and 3,930,867. Not only selection of a reduction sensitizer but also improvements in a reduction sensitization method are described in JP-B-57-33572 and JP-B-58-1410 ("JP-B-" means examined Japanese patent application). In these disclosures, conventional reduction sensitizers are enumerated, and ascorbic acid is included therein. In these disclosures, however, a compound such as thiourea dioxide is considered to be preferable, and thiourea dioxide, silver ripening, and hydrazine are exemplified. Therefore, preferable properties of an ascorbic acid compound as a reduction sensitizer have not been yet found. Improvements are also disclosed in JP-A-57-179835 ("JP-A-" means unexamined published Japanese patent application).
In order to realize reduction sensitization, a problem of storage stability must be solved. Techniques of improving storage stability of an emulsion subjected to reduction sensitization are disclosed in JP-A-57-82831 and JP-A-60-178445, but improvements have not reached a sufficient level. Regardless of a number of studies as described above, an increase in sensitivity is insufficient as compared with that obtained in hydrogen sensitization in which a light sensitive material is treated with hydrogen gas in a vacuum. This is reported by Moisar et al. in "Journal of Imaging Science", Vol. 29, P. 233 (1985). A demand has arisen for also improving in storage stability of a light-sensitive material containing a reduction-sensitized emulsion.
The conventional techniques of reduction sensitization do not satisfy a recent demand for high sensitivity and high image quality of a photographic light-sensitive material. This is because, firstly, variations in sensitivity and fogging density are large when a light-sensitive material containing an emulsion subjected to reduction sensitization is stored. Secondly, an increase in sensitivity obtained by reduction sensitization is insufficient.